Examining the tips of the spark plugs as they are removed can reveal a great deal about the health and performance of an engine. The appearance and color of the deposits can reveal other problems that may need fixing:

Normal deposits Light brown or tan colored.

Fuel fouled spark plug Black fluffy carbon deposits indicate an overly rich fuel mixture or possibly a weak spark. Check for such things as a stuck choke, a heavy or misadjusted carburetor float, a leaky needle valve in the carburetor, leaky injectors, low coil output or high resistance in the plug wires.

Wet spark plug A wet spark plug means the plug has not been firing. If not due to engine flooding, the problem may be a bad ignition cable (excessive resistance, shorted or arcing). But wet fouling can also be caused by dirt or moisture on the outside of the plug that provides a conductive path to ground, or by an internal crack in the ceramic insulator that shorts the plug to ground.

Oil fouled spark plug Heavy black deposits with an oily appearance. These are the result of oil entering in the combustion chamber, probably past worn valve guides, guide seals or rings. Switching to a hotter plug may help prolong plug life somewhat, but no spark plug will survive long under such conditions. The only permanent cure to this condition is to fix the oil consumption problem.

Glazed spark plug Yellowish melted appearing deposits on the insulator tip that result from high temperature operation. The engine may be running too hot (check for cooling problems), the EGR valve may be inoperative and/or the heat range of the plug may be too hot for the application. Switching to a cooler plug may be necessary if no other problems are found.

Damaged plug If the electrodes have been smashed flat or broken, somebody put the wrong plug in the engine. A plug that protrudes too far into the combustion chamber may hit the piston or a valve. Always follow the plug manufacturers application recommendations when selecting replacement plugs to prevent this kind of problem.

Overheating If the spark plug insulator is blistered, white and free from deposits, something is making the plug run too hot. If the heat range is not too hot for the application, check for cooling problems, incorrect ignition timing or a lean fuel mixture.

Melted electrode A symptom of severe preignition. The spark plug has been running too hot for a long time (see overheating above). This can be very damaging and may burn a hole through the top of a piston!

Detonation If the insulator is split or chipped, detonation (spark knock) may be occurring in the engine. The underlying cause here might be an inoperative EGR valve, overadvanced ignition timing, excessive compression due to accumulated deposits in the combustion chamber, or engine overheating.